It is well known that electrical equipment will generate electromagnetic emissions, including radio frequency emissions of radio frequency interference (“RFI”). RFI is an electromagnetic disturbance which is generated by electrical apparatus and can be received by and effect other equipment. In power supplies, the RFI is most commonly generated by the switching devices. Steep voltage steps and high switching frequencies increase the high frequency content of RFI disturbances. The RFI disturbances can couple to other components in the power supply, producing noise radiation over a broad frequency spectrum.
International basic emissions standards have been prepared by the “Comité international spécial des perturbations radioélectriques—International special committee of radio interferences” or “CISPR” and adopted by national and international authorities. CISPR 11 applies to industrial, scientific and medical electrical equipment operating in the frequency range 0 Hz to 400 GHz and to domestic and similar appliances designed to generate and/or use locally radio-frequency energy. CISPR 11 also cover emission requirements related to radio-frequency (RF) disturbances in the frequency range of 9 kHz to 400 GHz. The graph of FIG. 10 generally depicts the CISPR 11 disturbance field strength limits.
The CISPR 11 limits are applicable to inductive heating devices, such as inductive heated soldering stations that fall under the category of industrial equipment. Similar standards apply to inductive cooking system. RFI emissions from these types of inductive heating systems is a concern as the inductive heating systems include a power supply that provides a high frequency alternating (AC) current to a heater coil assembly. Also, in order to control temperature, the high frequency AC current may be pulsed on and off, providing very steep voltage steps. Accordingly, the circuit design for inductive heating devices disclosed herein contemplates isolating the causes of RFI emissions so as to attempt to minimize the RFI disturbances.